Computers widely used today are capable of storing and rapidly manipulating large amounts of data. Typically, such data is stored on some type of magnetic recording medium consisting of a thin magnetic film supported by a substrate. A relatively simple example of such media are magnetic tapes wherein the magnetic film is placed on a flexible polymer film.
The introduction of small computers in recent years for individual users, the so-called "personal computer" or "PC", has generated a need for other types of magnetic data storage needs. Such computers can employ a so-called "floppy disk" for data storage where the magnetic film is placed on a small circular disk substrate that is somewhat rigid, but still flexible. Typically, these computers also employ, as an important component thereof, a so-called "hard disk" drive wherein a relatively large amount of data can be stored on one or more magnetic disks, each of which comprise a magnetic film supported by a rigid, non-flexible disk substrate.
Research efforts have recently focused on improving magnetic data storage technology by trying to reduce the size of the hard disk (to decrease computer size) and, at the same time, attempting to increase the amount of data that can be stored on the disk. Information stored on a magnetic medium is read by the computer using a recording (read-write) head or a read-only head which can "float", i.e., pass, directly over the surface of the magnetic medium and thereby "read" information stored magnetically thereon. Information stored on the medium is measured in terms of bits of data per unit area, typically referred to as areal density. Factors which affect areal density are the thickness, coercivity, magnetic axis orientation, and crystalline texture of the magnetic film, as well as the height at which the recording head floats over the medium. In general, it is advantageous to have the head float as close as possible to the surface of the medium and at a constant height, preferably as low as about 50 Angstroms (.ANG.), since this helps maximize useful storage density.
Substrates conventionally used in hard disk storage media have been based on an aluminum or aluminum-alloy core which is coated with a thin electrodeposited nickel-phosphorous (NiP) layer and finally a thin magnetic metal film, typically of a cobalt-chromium alloy. An example of such storage media is described in U.S. Pat. No. 4,069,360. While such substrates have been successfully used in the past, they are limited in the amount of information that can be stored thereon due to characteristics of the aluminum-NiP substrate.
For example, one problem with such conventional substrates concerns a tendency for aluminum-based disks to deform, i.e., warp, when subjected to the high temperatures necessary to form a suitable magnetic film on the disk. The NiP coating also has a relatively low melting point which similarly impairs the flatness of the intermediate substrate when it is heated in subsequent processing steps. Due to these problems, manufacturing processes presently used to make media based on an aluminum-NiP substrate yield a significant number of defective products that cannot be sold commercially and are typically discarded. The high rejection rate appreciably adds to the cost for such products.
Aluminum and aluminum alloys can be relatively soft materials and, therefore, the surface of these substrates is susceptible to damage during subsequent processing steps and also from improper handling. Further, due to the crystal structure of aluminum, it is difficult to obtain a smooth, mirror-like finish and thereby minimize surface irregularities thereon, which can be thought of simplistically as "peaks" or "valleys" Aluminum or aluminum alloys also can, following high temperature exposure, form intermetallic inclusions which further roughen the surface. These surface irregularities are undesirable, since the magnetic film deposited on the substrate is typically on the order of only about a few hundred angstroms (.ANG.) thick or less and, therefore, any underlying substrate surface irregularities may adversely effect the magnetic axis orientation and texture of the overlying magnetic layer.
The above-described problems with conventional aluminum-based substrates all interfere with the need to have the head float as close as possible to the disk surface. If the surface is uneven due to warpage, or alternatively, has a significant amount of surface peaks and valleys, then the substrate will have a high average surface roughness (Ra). Since the head floats at a height related to this average surface roughness, these problems can limit the amount of information which can be written onto and read from the substrate.
Due to these limitations with conventional aluminum-based substrates, a number of materials have been recently proposed as replacements, such as glass, glass-coated alumina, carbon, silicon nitride and silicon carbide. Glass suffers from a similar strength and rigidity problem, since it tends to also deform when exposed to high temperature. Silicon nitride and silicon carbide have much better strength, toughness, and chemical resistance, especially at high temperatures, but are inherently porous to some extent and therefore, by themselves, have an undesirable surface roughness associated with them. Carbon is disadvantageous due to its porosity, low elastic modulus, and difficulties involved with bonding thin layers of materials on the surface thereof.
Others have previously attempted to employ silicon carbide as a substrate for magnetic storage disks. Japanese patent publication JP 60-229224 discloses a magnetic disc substrate consisting of silicon carbide which is coated with a thin sputtered film of Al.sub.2 O.sub.3, SiO.sub.2 and/or Si.sub.3 N.sub.4. While the inventors of this substrate purport to provide a poreless and strainless coating on the silicon carbide, these coatings are brittle and of relatively low strength. It is believed that such substrates are susceptible to undesirable chipping and surface damage from handling during manufacture and use. Also, such films are relatively non-conductive and, therefore, tend to build up static electricity during use which is generally undesirable for electronic devices, such as computers.
U.S. Pat. No. 4,598,017 discloses a composite magnetic disk which incorporates a reaction-bonded, silicon carbide substrate. The silicon carbide substrate is initially treated in a siliciding step wherein silicon is said to be deposited into pores on the surface of the silicon carbide and eventually a silicon layer is formed. Thereafter, the surface of the silicon is polished to a final surface roughness of 25 nm (250 .ANG.) Ra, and if this is not possible, the siliciding and polishing steps are repeated. A magnetic layer is formed on the silicon and the substrate is thereafter bonded to an annular, polymeric core.
While the inventors of this composite disk purport to improve the surface finish of reaction-bonded silicon carbide, a surface roughness of 25 nm Ra is not sufficiently smooth to allow use of the low head heights and extremely thin magnetic layers necessary for the high density data storage needs presently of interest to industry. Furthermore, the silicon layer deposited is also inherently brittle and, thus, it is believed to be susceptible to undesirable chipping during manufacture and use. The silicon layer is also relatively non-conductive and subject to the static electricity problem previously mentioned.
As a result, it is desirable to develop a substrate which has good strength, high thermal conductivity, chemical attack resistance, toughness, electrical conductivity, and relatively low coefficient of thermal expansion under conditions in which magnetic media are subjected to during manufacture and use. It would also be desirable for such a substrate to be capable of being polished to a very fine surface finish. Such a substrate material could then be used to produce a magnetic storage medium which allows a recording or read-only head to float at a very low head height so as to maximize areal density.